To produce tagatose from galactose two possible approaches are currently known: enzymatic or chemical. For the enzymatic approach, the use of arabinose isomerase is known, while for the chemical approach the use of strong bases is necessary in presence of an agent capable of complexing with tagatose to preserve the structure thereof so that it is not subject to degradation in the highly alkaline environment.
WO 2003/08617 describes the preparation of tagatose through enzymatic isomerisation of galactose mediated by arabinose isomerase with conversions in the order of 40% in around 24 hours; the method also provides for chromatography separation of galactose and tagatose.
WO 2008/072864 describes a method for preparing tagatose from galactose using an enzymatic approach with the use of arabinose isomerase and in presence of borates; the isomerisation reaction, performed at 60° C. and pH 8.5 after 10 hours produces a conversion of 77%.
Processes for producing tagatose using the enzymatic approach have the great advantage of giving rise to selective and therefore “clean” reactions (without degradation products) but by using arabinose isomerase, the natural substrate of which is not galactose, tagatose production takes place with low conversions and long reaction times and therefore with low productivity on an industrial scale. Moreover, the low conversion of galactose to tagatose makes chromatography purification of the latter necessary, a process that is not economically advantageous on an industrial scale.
U.S. Pat. No. 4,273,922 describes isomerisation of galactose to tagatose with a conversion of 52% using a chemical approach in presence of boric acid and a tertiary or quaternary amine in quantities sufficient to produce a pH between 9 and 11 for 6 hours at 70° C. Removal of the residual boric acid at the end of the reaction in however difficult.
WO 92/12263 describes chemical isomerisation of galactose to tagatose; the reaction is performed at pH 12.5 at room temperature for around 2 hours in presence of Ca(OH)2 and CaCl2, thus obtaining conversions of around 85% upon reaching which the reaction is neutralised by CO2 insufflation so as to obtain precipitation of CaCO3, which is removed by filtration.
CN 1985624 describes isomerisation of galactose to tagatose, without the addition of bases, in presence of NaAlO2 at 10-37° C. for 1-3 hours, after cooling the solution is acidified with the addition of H2SO4 which produces a precipitate containing aluminate which is removed thus providing a tagatose solution.
Prior art processes provide, after isomerisation, a tagatose solution contaminated by albeit small quantities of galactose which are however difficult to remove by crystallisation as galactose is less soluble than tagatose and therefore require purification processes that are not economically advantageous, such as chromatography.
The object of the present invention is to provide a process, which is also applicable on an industrial scale, for preparing tagatose with high degrees of purity, using low cost starting materials, by chemical isomerisation of galactose and by removing residual galactose in an economically advantageous and simple manner that does not require the use of costly and complicated purification techniques.